In the process of completing most oil wells, a string of conduit is placed into the open borehole and cemented in place by pumping a cement slurry down the annulus between the casing and the borehole wall.
The conduit or "casing" is run in the borehole in standard length sections joined together by threaded collars. The casing ends are threaded into the collars but the two ends are not in abutting relationship with each other leaving an axial annular space within each collar.
After the casing is cemented in the well, the casing may be perforated at the producing locations which have been located by sophisticated electronic, sonic, and radioactive logging methods. When lowering a perforating tool into the wellbore, it is desirable to have a quick, easy method to monitor how far down the borehole the tool has traveled so that it can be placed precisely adjacent the desired perforating location.
This can be accomplished roughtly by measuring the wireline or tubing carrying the tool into the well. To be more accurate, the operator needs to be able to correlate the depth with the well log. This he can do if he can know when the tool is at a specific casing collar near the formation to be perforated.
This knowledge can be ascertained by the use of a casing collar indicator. The prior art devices utilize electronic and magnetic sensing means to attempt to locate the collars. Other types utilize mechanical indicators which have fingers or blocks that must slide outward into engagement in the collar. These devices suffer reliability deficiencies because of their complexity and inability to distinguish collars from other types of discontinuities in the casing string.
The present invention provides a much less complex and more reliable apparatus to indicate when the tool has engaged a collar. Used in connjunction with a weight indicator on the tubing string, the present invention gives a precise indication of when the collar has been engaged.
The tool is also advantageous when lowering into the wellbore, a production or workover string containing packers, valves and other tools which need to be placed in close proximity to a producing formation.